*SOLVED* - Blacktop 20V High Idle

[70XIN]

Hey guys,

I have a blacktop 20V in an AE86. It has a stock ECU. It's idling at about 2000rpm 'cold start', and 1500 at 'normal driving'.

I have cleaned the ISC valve.

When i remove the ISC valve from the engine and block off the 'idle tube' going up behind the head with my finger, the car idles perfectly at 800-900rpm. I have inspected every vacuum line, and i can't find any leaks.

With the ISC valve removed, and that tube blocked off, shouldn't the car stall? Should the ITB's be sucking in air at idle? Could the vacuum "thingy" that sits under the ITB's be screwing with things?

Love,
Seamus

[slydar]

sounds like you have a vacuum leak or incorrectly plumped vacuum hoses. the throttles are meant to pass air even when shut. something to note. the icv valve is contolled by 2 opposing magnates. so it defaults to a neutral/medium setting if unplugged etc. this should still result in a normal (if possibly a ting bit rough) idle.

pretty sure the vacum thingy is part of the system that cushions the throttles when you let them snap shut. stopping the car from stalling under that circumstance. it should be left in place.

wait for an to reply or pm him.

[Anthony]

prob a/c request. usually disconnected is fine, but on a blacktop stock ecu, if its left floating, the ecu assumes ac is on, not off. adds a few hundred to the base idle.

could be vac too. the lines for the power steer idle up and ac idle up sometimes get misplaced.

EDIT: OK, read properly Yes, the enigne should stall or at least try and run very very low with the ISCV blocked. does the dashpot(the diaphragm thing under the throttle stop) have a vac line connected? Up next to where the big hose leads in to the top rail from the ISCV, there are two other smaller ports. Are they blocked?

[70XIN]

Okay the vacuum line layout is as follows:

A goes to A

B goes to B

C is blocked off (supposed to go to VSV valve which is connected to charcoal canister? i have no VSV valve, so my charcoal canister currently 'vents to atmosphere', and the vacuum line 'C' is blocked off with a bolt)

D is a tiny tiny little breather filter

E goes to the dashpot





Now there's definitely air getting in through the ITB's at idle, and blocking them up as well as i can with my hands seem to make little difference if any to the idle.

[Anthony]

Looks like a clean car!!

I take it it has aircon? Seems they have tried to add a second switched idle up on the dashpot. Not sure on the reasoning for this. I would try connecting the dashpot to vac directly. You dont normally do that from "A", but rather from a vac port on the underside of the throttles. Or you can push the dashpot and beat the spring. if the idle drops from pushing it down a bit (simulating vacuum pulling the dashpot stop inwards) then I'd say you're on the right track.

[70XIN]

Yep has aircon and is very clean! Just have to fix this magical idle issue. OH i forgot to add (don't know if it's important), but the car doesn't have an o2 sensor yet. There's wiring and a plug for it, but the previous owner just had a 1-wire sensor dangling around in the air. I have ordered a new 4-wire sensor and it will be here in a day or two.

I tried pushing down the dashpot rod by hand, to simulate the vacuum, and no such luck it didn't have any effect

[70XIN]

I also ran line "A" (from the first picture) straight to the dashpot and it made no difference

[Anthony]

OK. well blacktop ecu's will only behave nicely if every last thing is perfect, including electrical load inputs (there are 3 from memory), ac request as mentioned, and especially the o2 sensor (including the heater circuit). If these arent all hooked up, even grounded permanently if theyre not needed, then the ECU wont perform very well. I know you've taken the ISCV out of the equation here, but keep that in mind anyway. What was done with the brake booster connections? There are two normally, one from ITB#1, and one from #4. If these are teed together, each one needs a check valve, a single check valve after the tee doesnt work out that well.

[70XIN]

Okay, i'm keeping both the o2 sensor and ISCV in mind. I should be able to write off the o2 sensor in the next day or so

The brake booster is using ITB #1. The brake booster vacuum line has both check valves on the one line haha. ITB #4 has been blocked off with a bolt as far as i can see.

Also - can the water temp sensor screw up the idle too? Mine looked pretty rough (i was fixing a tiny leak at the back of the head), i have a spare which i can try out tomorrow too.

[Anthony]

Well it can, but it can only really influence it through the ISCV.

These motors run open loop idle control too. So if you're idling at 1200 already due some other reason and the ECU gets the AC req signal, it doesnt "see" that the idle is already high, it just adds 5% idle effort to the ISCV drive regardless, taking the high idle even higher. If the 3 pipes at the end of the top rail (A, B and the big ISCV pipe next to C in your pic) are all blocked, and the motor still idles, then it has to be getting too much air through the throttles themselves, either via the throttle stop, or via the mixture adjustment screws. Hopefully they didnt stuff around with the mixture screws too much, so I guess maybe just crack the lock nut on the throttle stop adjustment and wind that in a bit.